Flipped vertical field-effect-transistor

ABSTRACT

Various embodiments disclose a method for fabricating vertical transistors. In one embodiment, a structure is formed comprising at least a first substrate, an insulator layer on the substrate, a first doped layer on the insulator layer, at least one fin structure in contact with the doped layer, a dielectric layer surrounding a portion of the fin structure, a gate layer on the dielectric layer, a second doped layer in contact with the fin structure, a first contact area in contact with the second doped layer, and at least a first interconnect in contact with the first contact area. The structure is flipped bonded to a second substrate. The first substrate and the insulator layer are removed to expose the first doped layer. A second contact area is formed in contact with the first doped layer. At least a second interconnect is formed in contact with the second contact area.

BACKGROUND OF THE INVENTION

The present disclosure generally relates to the field of semiconductors,and more particularly relates to vertical field-effect-transistors.

Vertical transistors are a promising option for technology scaling for 5nm CMOS technology and beyond. Vertical field-effect-transistors (FETs)mainly comprise a bottom S/D, a vertical fin or nanowire channel, a topS/D, gate contact, and metal contacts to top and bottom S/D. Additionalarea is consumed in order to make contact to the bottom S/D so that thebottom S/D can be electrically accessed from the top resulting inincreased device footprint and reduced device packing density.

SUMMARY OF THE INVENTION

In one embodiment, a method for fabricating vertical transistors. Themethod comprises forming a structure comprising at least a firstsubstrate, an insulator layer on the substrate, a first doped layer onthe insulator layer, at least one fin structure in contact with thedoped layer, a dielectric layer surrounding a portion of the finstructure, a gate layer on the dielectric layer, a second doped layer incontact with the fin structure, a first contact area in contact with thesecond doped layer, and at least a first interconnect in contact withthe first contact area. The structure is flipped and then bonded to asecond substrate. The first substrate and the insulator layer areremoved to expose the first doped layer. A second contact area is formedin contact with the first doped layer. At least a second interconnect isformed in contact with the second contact area.

In another embodiment, a vertical transistor is disclosed. The verticaltransistor comprises a substrate. A first interconnect is formed overthe substrate and a contact area. A first doped layer is formed situatedover the contact area. At least one fin structure is in contact with thefirst doped layer. A dielectric layer is in contact with a portion ofthe fin structure. A metal gate layer is in contact with the dielectriclayer. A second doped layer is in contact with the fin structure. Atleast a second interconnect is formed over the second doped layer.

In yet another embodiment, an integrated circuit is disclosed. Theintegrated circuit comprises at least one vertical transistor. Thevertical transistor comprises a substrate. A first interconnect isformed over the substrate and a contact area. A first doped layer isformed situated over the contact area. At least one fin structure is incontact with the first doped layer. A dielectric layer is in contactwith a portion of the fin structure. A metal gate layer is in contactwith the dielectric layer. A second doped layer is in contact with thefin structure. At least a second interconnect is formed over the seconddoped layer.

In a further embodiment, a method for forming inner-cell connectionswithin a circuit is disclosed. The method comprises coupling a firstvertical field-effect-transistor (pFET) to an output line at a top-levelof the circuit. A determination is made as to whether a second verticalFET is connected to the first vertical FET. In response to a secondvertical FET failing to be connected to the first vertical FET, thefirst vertical FET is coupled to a bottom rail situated at a bottomlevel of the circuit. In response to a second vertical FET beingconnected to the first vertical FET, the first vertical FET and thesecond vertical FET are coupled to each other at the bottom level of thecircuit. A determination is made as to whether at least a third verticalFET is connected to the second vertical FET. In response to at least athird vertical FET failing to be connected to the second vertical FET,the second vertical FET is coupled to a top rail situated at the toplevel of the circuit. The top level and bottom level are absent anydirect interconnects there between. In response to at least a thirdvertical FET being connected to the second vertical FET, the secondvertical FET and the third vertical FET are coupled to each other at thetop level of the circuit.

In another embodiment, a circuit is disclosed. The circuit comprises atop supply rail and a top ground rail disposed within a top level of thecircuit. A bottom supply rail and a bottom ground rail are disposedwithin a bottom level of the circuit. At least one input line isdisposed within a middle level of the circuit. An output line isdisposed within the top level of the circuit. At least one p-typevertical field-effect-transistor (FET) is coupled to one of the bottomsupply rail and the top supply rail. At least one n-type vertical FETcoupled to one of the bottom ground rail and the top ground rail.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures where like reference numerals refer toidentical or functionally similar elements throughout the separateviews, and which together with the detailed description below areincorporated in and form part of the specification, serve to furtherillustrate various embodiments and to explain various principles andadvantages all in accordance with the present disclosure, in which:

FIG. 1 is a cross-sectional view of an initial semiconductor structureaccording to one embodiment of the present disclosure;

FIG. 2 is a cross-sectional view of the semiconductor structure afterfin structures have been formed according to one embodiment of thepresent disclosure;

FIG. 3 is a cross-sectional view of the semiconductor structure after afirst spacer layer has been formed according to one embodiment of thepresent disclosure;

FIG. 4 is a cross-sectional view of the semiconductor structure afterthe first spacer layer and first doped layer have been etched accordingto one embodiment of the present disclosure;

FIG. 5 is a cross-sectional view of the semiconductor structure after adielectric layer has been formed according to one embodiment of thepresent disclosure;

FIG. 6 is a cross-sectional view of the semiconductor structure after ahigh-k dielectric layer and metal gate layer have been formed accordingto one embodiment of the present disclosure;

FIG. 7 is a cross-sectional view of the semiconductor structure a secondspacer layer has been formed according to one embodiment of the presentdisclosure;

FIG. 8 is a cross-sectional view of the semiconductor structure a seconddoped layer has been formed according to one embodiment of the presentdisclosure;

FIG. 9 is a cross-sectional view of the semiconductor structure after afirst contact area has been formed according to one embodiment of thepresent disclosure;

FIG. 10 is a cross-sectional view of the semiconductor structure after afirst set of interconnects have been formed according to one embodimentof the present disclosure;

FIG. 11 is a cross-sectional view of the semiconductor structure afterthe structure has been flipped and bonded to a new substrate accordingto one embodiment of the present disclosure;

FIG. 12 is a cross-sectional view of the semiconductor structure afterthe original substrate and insulator layer have been removed accordingto one embodiment of the present disclosure;

FIG. 13 is a cross-sectional view of the semiconductor structure after asecond set of interconnects and a dielectric layer have been formedaccording to one embodiment of the present disclosure;

FIG. 14 is a schematic illustrating an overall example of inner-cellconnections for a circuit according to one embodiment of the presentdisclosure;

FIG. 15 is a schematic illustrating one example of inner-cellconnections for a NAND gate according to one embodiment of the presentdisclosure;

FIG. 16 is a schematic illustrating one example of inner-cellconnections for an inverter according to one embodiment of the presentdisclosure;

FIG. 17 is a schematic illustrating one example of inner-cellconnections for a two-input NAND gate according to one embodiment of thepresent disclosure;

FIG. 18 is a schematic illustrating one example of inner-cellconnections for a two-input NOR gate according to one embodiment of thepresent disclosure;

FIG. 19A is a top-view schematic of two-input NAND gate illustratinginner-cell connections according to one embodiment of the presentdisclosure;

FIG. 19B is a bottom-view schematic of two-input NAND gate illustratinginner-cell connections according to one embodiment of the presentdisclosure;

FIG. 20 is a schematic illustrating one example of inter-cellconnections between different gates according to one embodiment of thepresent disclosure;

FIG. 21 is an operational flow diagram illustrating one process forforming a vertical transistors according to one embodiment of thepresent disclosure; and

FIGS. 22 and 23 are an operational flow diagrams illustrating oneprocess for forming inner-cell connections according to one embodimentof the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the present disclosure will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present disclosure.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

Referring now to the drawings in which like numerals represent the sameof similar elements, FIGS. 1-22 illustrate various processes forfabricating flipped vertical field-effect-transistors (FETs) and theirinterconnects. FIG. 1 shows a partially fabricated semiconductorstructure 100 comprising a substrate 102, an insulator layer 104, adoped layer 106 (also referred to herein as “source/drain layer 106”),and a channel layer 108. The thickness of the substrate 102 can be, forexample, from 50 microns to 1,000 microns, although lesser and greaterthicknesses can be employed as well. The substrate 102 can be singlecrystalline and or a bulk substrate, a semiconductor-on-insulator (SOI)substrate, or a hybrid substrate. The insulator layer 104 comprises adielectric material such as silicon oxide, silicon nitride, siliconoxynitride, or any combination thereof. In one embodiment, the insulatorlayer 104 is a buried oxide (BOX) layer.

The substrate 102 can be appropriately doped either with p-type dopantatoms or with n-type dopant atoms, or the material can be substantiallyundoped (intrinsic). The dopant concentration of the substrate 102 canbe from 1.0×10¹⁵/cm³ to 1.0×10¹⁹/cm³, and in one embodiment, is from1.0×10¹⁶ cm³ to 3.0×10¹⁸/cm³, although lesser and greater dopantconcentrations are applicable as well. An optional counter-doped layer(not shown) can be formed on and in contact with the substrate 102 (orburied insulator layer if formed). The counter-doped layer, in oneembodiment, is formed by an epitaxial growth of a semiconductormaterial. The counter-doped layer may be implanted with dopants andannealed using, for example, rapid thermal anneal. Alternatively, thecounter-doped layer can be doped in-situ during the epitaxial growth.The purpose of the counter-doped layer is to provide an isolationbetween one transistor and the next transistor.

The source/drain layer 106 is formed on and in contact with theinsulator layer 104 (or counter-doped layer if formed). The source/drainlayer 106 can be, for example, an n++ or a p++ doped region of thesubstrate 102 and can have a thickness in a range of, for example, about10 nm to about 200 nm. However, other thicknesses are applicable aswell. The source/drain layer 106 can be formed by epitaxial growth. Thechannel layer 108, in one embodiment, comprises a channel material. Thechannel material can be formed using an epitaxy process that grows amaterial up from the source layer 104. The channel material can beundoped or doped with either p-type or n-type dopants through ionimplantation, plasma doping, or gas phase doping. P-type transistors areproduced by doping the channel material with elements from group III ofthe periodic table (e.g., boron, aluminum, gallium, or indium). As anexample, the dopant can be boron in a concentration ranging from 1×10E18atoms/cm3 to 2×10E21 atoms/cm3. N-type transistors are produced bydoping the channel material with elements from group V of the periodictable (e.g., phosphorus, antimony, or arsenic). As an example, thedopant can be phosphorus in a concentration ranging from 1×10E14atoms/cm3 to 1×10E20 atoms/cm3.

FIG. 2 shows the semiconductor device 100 after fin structures 202, 204have been formed in the channel layer 108. The fins 202, 204 are formed,in one embodiment, by forming an etch-stop capping layer (not shown)onto the channel material through, for example, deposition. Theetch-stop capping layer, in one embodiment, may be made ofsilicon-nitride although other material suitable in providing etch-stopfunction may be used as well. One or more fin structures 202, 204 aresubsequently formed or etched out of the channel material to be on topof and in contact with the source/drain layer 106 through a processinvolving masking, using industry-standard lithographic techniques, anddirectionally etching the etch-stop capping layer and underneath channelmaterial. The directional etching process, for example areactive-ion-etching (RIE) process, stops on the insulating layer 104.After the RIE etching process, the photo-resist mask used in thelithographic etching process can be removed. The etch-stop capping layercan also be removed as well. It should be noted that although thefigures show two fin structures, embodiments of the present disclosureare

After the fins 202, 204, have been formed, bottom spacers 302 are formedin contact with each of the fins 202, 204, as shown in FIG. 3. Eachbottom spacer 302 contacts a top surface 304 of the source/drain layer106 and sidewalls 306, 308 of at least one fin 202, 204. In oneembodiment, the bottom spacers 302 comprise an insulating material (suchas silicon oxide, silicon nitride, silicon oxynitride, or a combinationof these) and can be formed using any conventional deposition processsuch as, for example, chemical vapor deposition (CVD) and subsequentetching techniques. The deposited spacer material is then subsequentlyetched to form the final spacer structures. In one embodiment, thespacers have a thickness of, for example, 3 nm to 30 nm.

FIG. 4 shows that once the bottom spacers 302 have been formed, aportion of the bottom spacers 302 and the source/drain layer 106 areetched using standard lithography and etching techniques. This processcreates trenches 402, 404, 406 exposing a top surface 408 of theinsulating layer 104 and sidewalls 410, 412 of the source/drain layer106 and bottom spacers 302. An interlayer dielectric 502 is thendeposited and followed by chemical mechanical planarization (CMP)process and followed by etching back. The interlayer dielectric 502fills the trenches 402, 404, 406 and comprises a top surface 504 that isco-planar with the top surface 506 of the bottom spacers 302. Theinterlayer dielectric 502 may comprise SiO2, Si3N4, SiOxNy, SiC, SiCO,SiCOH, and SiCH compounds; the above-mentioned silicon-based materialswith some or all of the Si replaced by Ge; carbon-doped oxides;inorganic oxides; inorganic polymers; hybrid polymers; organic polymerssuch as polyamides or SiLK™; other carbon-base materials;organo-inorganic materials such as spin-on glasses andsilsesquioxane-based materials; and diamond-like carbon (DLC, also knownas amorphous hydrogenated carbon, α-C:H). Additional choices for theblanket dielectric include any of the aforementioned materials in porousform, or in a form that changes during processing to or from beingporous and/or permeable to being non-porous and/or non-permeable.

A high-k dielectric material is then blanket deposited over the entirestructure 100, for example by CVD (chemical vapor deposition), PECVD(plasma enhanced chemical vapor deposition), or ALD (Atomic layerdeposition). Excessive high-k gate dielectric material is removed, forexample, by polishing such as chemically mechanical polishing (CMP)and/or etching to form high-k gate dielectric layers 602, 604, as shownin FIG. 6. Each high-k gate dielectric layer 602, 604 is formed on andin contact with sidewalls 306, 308 of a fin 202, 204 and the top surface506 of the bottom spacers 302. The high-k gate dielectric layers 602,604 comprise a top surface 606, 608 that is co-planar with a top surface610, 612 of the fins 202, 204. Examples of high-k materials include butare not limited to metal oxides such as hafnium oxide, hafnium siliconoxide, hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminumoxide, zirconium oxide, zirconium silicon oxide, zirconium siliconoxynitride, tantalum oxide, titanium oxide, barium strontium titaniumoxide, barium titanium oxide, strontium titanium oxide, yttrium oxide,aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. Thehigh-k layers 602, 604 may further include dopants such as lanthanum oraluminum.

In one embodiment, the high-k layers 602, 604 are part of a layercomprising a work function metal layer (not shown). In one embodiment,the work function metal layers are formed after and conformal to thehigh-k layers 602, 604 employing CVD, sputtering, or plating. The workfunction metal layers comprise one or more metals having a functionsuitable to tune the work function of nFETs or pFETs. Exemplary firstmetals that can be employed in the work function metal layer include,but are not limited to La, Ti and Ta. The thickness of the work functionmetal layers can be from 3 nm to 15 nm, although lesser and greaterthicknesses can also be employed.

FIG. 6 further shows that a metal gate 614 is formed around the fins202, 204. For example, a metal gate material can be deposited by atomiclayer deposition (ALD) or physical vapor deposition (PVD). In oneembodiment, the metal gate 614 is a continuous metal gate that wrapsaround both fins 202, 204. The metal gate 614 contacts the outersidewalls 615, 616 of the high-k gate dielectric layers 602, 604; thetop surface of the remaining portion 618 of the bottom spacers 302; anda top surface 620 of a portion 622 of the interlayer dielectric 502formed between the fins 202, 204. A top surface 624 of the metal gate614 is co-planar with the top surface 606, 608 of the high-k dielectriclayers 602, 604 and the top surface 610, 612 of the fins 202, 204. Inone embodiment, the metal gate 614 comprises, for example, tungsten.Additional interlayer dielectric material is added to the interlayerdielectric 502 such that first and second portions 621, 623 of theinterlayer dielectric 502 comprise a top surface 504 that is co-planarwith the top surface 624 of the metal gate 614 and the top surface 606,608 of the high-k dielectric layers 602, 604, whereas a third portion622 of the interlayer dielectric 502 formed between the fins 202, 204comprises a top surface 620 that is co-planer with a top-surface 506 ofthe bottom spacers 302.

Top spacers 702, 704 are then formed on the structure 100, as shown inFIG. 7. The top spacers 702, 704 comprise a bottom surface 706, 708 thatcontacts the top surface 624 of the metal gate 614; the top surface 606,608 of the high-k dielectric layers 602, 604; and portions of thesidewalls 306, 308 of the fins 202, 204 that are above the metal gate614 and high-k dielectric layers 602, 604. The top surface 710, 712 ofthe top spacers 702, 704 is co-planar with the top surfaces 610, 612 ofthe fins 202, 204. In one embodiment, the top spacers 702, 704 comprisethe same or different material as the bottom spacers 302. For example,the top spacers 702, 704 can comprise an insulating material (such assilicon oxide, silicon nitride, silicon oxynitride, or a combination ofthese) and can be formed using any conventional deposition process suchas, for example, chemical vapor deposition (CVD) and subsequent etchingtechniques. The deposited spacer material is then subsequently etched toform the final spacer structures.

A doped layer 802, 804 (also referred to herein as “source/drain layer802, 804”) is then formed on and in contact with each the fins 202, 204and their respective top spacer 702, 704, as shown in FIG. 8. A bottomsurface 806, 808 of the source/drain layers 802, 804 contacts a topsurface 610, 612 of the fins 202, 204 and a top surface 710, 712 of thetop spacers 702, 704. The source/drain layers 802, 804 have a thicknessin a range of, for example, about 10 nm to about 200 nm. However, otherthicknesses are applicable as well. The source/drain layers 802, 804 canbe formed by epitaxial growth.

A metallization process is then performed to create contact areas 902,904 on and in contact with the source/drain layers 802, 804. Themetallization can involve CVD, PVD, ALD, or electroplating processes orsome combination of these processes. In one embodiment, the contactareas 902, 904 comprise a width that is substantially equal to the widthof the source/drain layers 802, 804 and a thickness for, for example, 10nm to 100 nm. Interlayer dielectric material is then added to theinterlayer dielectric 502 followed by chemical mechanical planarization(CMP) process such that the top surface 504 of the first and secondportions 621, 623 of the interlayer dielectric 502 is co-planar with thetop surface 906, 908 of the contact areas 902, 904.

The deposition of additional interlayer dielectric material also formsan interlayer dielectric 910 in between the fins 202 and above the thirdportion 622 of the interlayer dielectric 502. The additional interlayerdielectric 910 comprises a bottom surface 912 contacting the top surface624 of the portion 914 of the metal gate 614 between the fins 202, 204.Sidewalls 916 of the additional interlayer dielectric 910 contact thesidewalls 918, 920 of each top spacer layer 702, 704; sidewalls 922, 924of each source/drain layer 802, 804; and sidewalls 926, 928 of eachcontact areas 902, 904. A top surface 930 of the additional interlayerdielectric 910 is co-planar with the top surface of the contact areas902, 904.

In one embodiment, a layer of dielectric material 1002 can be blanketdeposited atop the entire structure 100 and planarized following theformation of the contact areas 902, 904, as shown in FIG. 10. Theblanket dielectric may be a silicon-based material, such as SiO2, Si3N4,SiOxNy, SiC, SiCO, SiCOH, and SiCH compounds; the above-mentionedsilicon-based materials with some or all of the Si replaced by Ge;carbon-doped oxides; inorganic oxides; inorganic polymers; hybridpolymers; organic polymers such as polyamides or SiLK™; othercarbon-based materials; organo-inorganic materials such as spin-onglasses and silsesquioxane-based materials; and diamond-like carbon(DLC, also known as amorphous hydrogenated carbon, α-C:H). Additionalchoices for the blanket dielectric include any of the aforementionedmaterials in porous form, or in a form that changes during processing toor from being porous and/or permeable to being non-porous and/ornon-permeable. The deposited dielectric 1002 is then patterned andetched to form via holes to the source/drain contact areas 902, 904. Insome embodiments, a via is also created within the additional interlayerdielectric 910 exposing a portion of the gate 614.

Following via formation, interconnects 1004, 1006 are formed bydepositing a conductive metal into the via holes using depositionmethods, such as CVD or plating. It should be noted that in embodimentswhere a via is formed through the additional interlayer dielectric 910,and an interconnect (not shown) is formed in contact with the gate 614.The conductive metal may include, but is not limited to, tungsten,copper, aluminum, silver, gold and alloys thereof. Separate supply (VDD)and ground rails (GND), not shown, are also formed and electricallycoupled to the interconnects 1004, 1006 via one or more embodimentsdiscussed below. The supply (VDD) and ground rails (GND) can be formedsimilar to the interconnects 1004, 1006 discussed above. In anotherembodiment, the interconnects 1004, 1006 serve as the supply (VDD) andground rails (GND).

After the drain interconnects 1004, 1006 have been formed, the structure100 is flipped/rotated 180 degrees, as shown in FIG. 11. After rotation,the bottom layer 1102 of the structure 100 comprises the dielectricmaterial 1002 and drain interconnects 1004, 1006 and the top layer 1104of the structure 100 comprises the original substrate 102. The bottomlayer 1102 is then bonded to a new substrate 1106. Any bonding techniquecan be utilized to bond the bottom layer 1102 to the substrate 1106. Thethickness of the new substrate 1106 can be, for example, from 50 micronsto 1,000 microns, although lesser and greater thicknesses can beemployed as well. The new substrate 1106 can be single crystalline andor a bulk substrate, a semiconductor-on-insulator (SOI) substrate, or ahybrid substrate.

FIG. 12 shows that the original substrate 102 and insulator layer 104and then removed exposing the source/drain layers 106. The originalsubstrate 102 and insulator layer 104 can be removed by, for example, aCMP process. A metallization process is then performed to create contactareas 1202, 1204 on and in contact with the source/drain layers 106. Themetallization can involve CVD, PVD, ALD, or electroplating processes orsome combination of these processes. In one embodiment, the contactareas 1202, 1204 comprise a width that is substantially equal to thewidth of the source/drain layers 106. Interlayer dielectric material isthen added to the first portion 621, second portion 623, and thirdportion 622 of the interlayer dielectric 502 followed by chemicalmechanical planarization (CMP) process such that a surface 1206, 1208,1210 of these portions 621, 622, 623 of the interlayer dielectric 502 isco-planar with the exposed surface 1212, 1214 of the contact areas 1202,1204.

In one embodiment, a layer of dielectric material 1302 can be blanketdeposited atop the entire structure 100 and planarized following theformation of the contact areas 1202, 1204, as shown in FIG. 13. Theblanket dielectric may a silicon-based material, such as SiO2, Si3N4,SiOxNy, SiC, SiCO, SiCOH, and SiCH compounds; the above-mentionedsilicon-based materials with some or all of the Si replaced by Ge;carbon-doped oxides; inorganic oxides; inorganic polymers; hybridpolymers; organic polymers such as polyamides or SiLK™; othercarbon-based materials; organo-inorganic materials such as spin-onglasses and silsesquioxane-based materials; and diamond-like carbon(DLC, also known as amorphous hydrogenated carbon, α-C:H). Additionalchoices for the blanket dielectric include any of the aforementionedmaterials in porous form, or in a form that changes during processing toor from being porous and/or permeable to being non-porous and/ornon-permeable. The deposited dielectric 1302 is then patterned andetched to form via holes to the contact areas 1202, 1204 and the gate614.

Following via formation, interconnects 1304, 1306, 1308 are formed bydepositing a conductive metal into the via holes using depositionmethods, such as CVD or plating. The conductive metal may include, butis not limited to, tungsten, copper, aluminum, silver, gold and alloysthereof. Supply (VDD) and ground rails (GND), not shown, are also formedand electrically coupled to the interconnects 1304, 1306, 1308 via oneor more embodiments discussed below.

The various embodiments discussed above form vertical FETs comprisingcontact areas 1202, 1202 at a top level 1310 of the structure 100 andcontact areas 902, 904 at a bottom level 1312 of the structure 100directly under the fins 202, 204. This configuration is advantageoussince it reduces the FET footprint and unit cell size, and furtherreduces series resistance induced by bottom contacts because the currentpath is shortened. In addition to the above, one or more embodimentsprovide various interconnect configurations for the vertical FETs. Theseinterconnect configurations avoid a connection between top and bottomcontacts in any logic cell unit, which can cause added middle of line(MOL) resistance, occupy space, and add process complexity. Theinterconnect configurations of one or more embodiments are realized bythe formation of supply (VDD) and ground (GND) rails on both the top andbottom levels of the vertical FETs.

With respect to inner-cell interconnects (connections within each typeof gate), any logic relation can be expressed as sum-of-product form.For example O=Ā+BC. Then, the pFET portion of a gate circuit comprisesparallel-connected branches each of which includes series-connectedpFETs. The nFET portion comprises series-connected branches each ofwhich includes parallel nFETs. One or more embodiments determine thelevel of each connection for pFETs and nFETS to supply (VDD) and ground(GND) rails, respectively, based on the following rules. It should benoted that the following rules are based on a top-level output. However,embodiments of the present disclosed are not limited to a top-leveloutput. It should be noted that throughout the following discussionreference is made to the top-level, middle-level, and bottom-level ofthe circuit. The middle-level is the level where the gate metal 614 andchannel 202, 204 are located. The bottom-level is defined to be betweenthe middle-level and the final support substrate (1106 in FIG. 13). Thetop-level is then on the opposite side of the bottom-level.

When determining the level of each connection, each series branch of thepFETS is traced starting from the output (at the top-level of the gatecircuit). The next connection after the output is to be made at thebottom-level of the circuit. The following connection is made at thetop-level of the circuit. The connections alternate between the bottomand top levels of the circuit until the supply (VDD) rail is reached.The pFETs are then coupled to the supply (VDD) rail at the currentlevel. Such procedure repeats for all pFET branches. The nFETs aretraced starting at the output where the first encountered nFETs areconnected at the bottom-level of the circuit. The following connectionis made at the top-level of the circuit. The connections alternatebetween the bottom and top levels of the circuit until the ground (GND)rail is reached. The nFETs are then coupled to the ground (GND) rail atthe current level.

For example, consider the gate circuit 1400 illustrated in FIG. 14. Thecircuit 1400 comprises supply (VDD) rails 1402 and ground (GND) rails1404 on both the top (T) and bottom (B) levels of the circuit. Thecircuit 1400 also comprises a pFET portion 1405 comprising a pluralityof pFETs 1406, 1408, 1410, and an nFET portion 1407 comprising aplurality of nFETs 1412, 1414, 1416. Each of the pFETs and nFETs arefabricated according the embodiments discussed above with respect toFIGS. 1-13. The pFET portion 1405 comprises parallel branches 1418, 1420each comprising the pFETs 1412, 1414, 1416, where two or more pFETs areconnected in series. The nFET portion 1407 comprises series-connectedbranches 1422, 1424 where 1422 comprises 1414 and 1416 connected inparallel and 1424 comprises 1412.

FIG. 14 further shows that the output 1426 is placed on the top level(T) of the circuit 1400. Starting within the pFET portion 1405 of thecircuit 1400 and tracing each branch 1418, 1420 starting from the output1426, the connection 1428, 1430 made between the drain terminal of thepFET A 1406 and the output 1426 and the drain terminal of pFET C 1410and the output at the top level (T) of the circuit 1400. The nextconnection 1432, 1434 within each parallel branch 1418, 1420 is made atthe bottom level (B) of the circuit 1400, and the following connection1436 is made at the top level of the (T) of the circuit 1400.

Any subsequent connections alternate between the bottom level (B) andthe top level of the (T) of the circuit 1400 until the supply rail (VDD)1402 is reached. When the supply rail (VDD) 1402 is reached, the currentpFET is connected to the supply rail (VDD) 1402 at the current level.For example, FIG. 14 shows that last connection 1436 in branch 1418 isbetween the source terminal of pFET B 1408 and the supply rail (VDD)1402. The previous connection (i.e., the connection between the drainterminal of pFET B 1408 and the source terminal of pFET C 1410) was madeat the bottom level (B). Therefore, the current level is at the toplevel of the (T) of the circuit 1400, and the connection 1436 betweenthe source terminal of pFET B 1408 and the supply rail (VDD) 1402 at thetop level of the circuit 1400.

Turning now to the nFET portion 1407 of the circuit 1400 and tracingeach branch 1422, 1424 starting from the output 1426, the connection1438 made between the drain terminal of nFET B′ 1414 and the output 1426and the connection 1440 between the drain terminal of nFET C′ 1416 andthe output 1426 is at the top level (T) of the circuit 1400. The nextconnection 1442, which connects the source terminals of the parallelnFETs 1414, 146 in the first branch 1422 and the drain terminal of thenFET 1412 in the second branch 1424, is made at the bottom level (B) ofthe circuit 1400. Any subsequent connections alternate between thebottom level (B) and the top level of the (T) of the circuit 1400 untilthe ground rail (GND) 1404 is reached. When the ground rail (GND) 1404is reached, the current nFET is connected to the ground rail (GND) 1404at the current level.

For example, the next connection 1442 after the output 1426, whichconnects the source terminals of the parallel nFETs 1414, 146 in thefirst branch 1422 and the drain terminal of the nFET 1412 in the secondbranch 1424, is made at the bottom level (B) of the circuit 1400. Then,the following connection 1444, which connects the source terminal ofnFET A′ 1412 to the ground rail (GND) 1404, is made at the top level ofthe circuit 1400. It should be noted that in one or more embodiments,the nFET-pFET pairs are aligned such that each pair can be connected bya gate line in layout. In addition, product-of-sum (POS) expression canbe done similarly, e.g., O=(C+B)Ā where the “nFET” and “pFET” areexchanged as compared to the SOP form.

With respect to inner-cell connections, connections made at the toplevel or bottom level depend on the number of transistors and the typeof gate. For example, FIG. 15 shows one example of a NAND gate 1500comprising a plurality of pFETs 1502 and a plurality of nFETs 1504. Thegate 1500 also comprises top/bottom supply rails (VDD) 1506, top/bottomground rails (GND) 1508, a plurality of inputs 1510, and an output 1512at the top level of the (T) of the gate 1500. In a NAND gate parallel(pMOS) FETs 1502 are all connected at the bottom level (B) to the bottomsupply rail (VDD) 1506. However, the connection of the series (nMOS)nFETs 1504 depends of the total number of nFETs within the circuit. Forexample, if the total number N of nFETs is even, the last nFET isconnected at the top level (T) to the top ground (GND) rail 1508. If thetotal number N of nFETs is odd then the last nFET is connected at thebottom level (B) level to the bottom ground (GND) rail.

Similar rules can apply to a NOR gate. For example, all parallel (nMOS)FETs are connected at the bottom level (B) to the bottom ground (GND)rail, while the connection level of the series (pMOS) FETs depends onthe total number of pFETs within the circuit. For example, if the totalnumber N of pFETs is even, the last pFET is connected at the top level(T) to the top supply (VDD) rail. If the total number N of pFETs is oddthen the last pFET is connected at the bottom level (B) level to thebottom supply (VDD) rail.

FIGS. 16-18 show examples of different gate circuits and theirinter-cell connections based on the interconnect rules discussed above.FIG. 16 shows one example of an inverter gate 1600 that comprises atop-level 1602, middle-level 1604, and bottom-level 1606. The top-level1602 comprises a top ground (GND) rail 1608, a top supply (VDD) rail1610, and an output 1612. The middle-level 1604 comprises a pFET 1614,an nFET 1616, and an input 1618. The bottom-level 1606 comprises abottom ground (GND) rail 1618 and a bottom supply (VDD) rail 1620.Following the rules discussed above with respect to FIGS. 14 and 15, thedrain terminal of the pFET 1614 and the drain terminal of the pFET 1614at the top level 1602 are coupled to the output 1612. The input 1618 iscoupled to the pFET 1614 and nFET 1616 at the middle level 1604. Thenext connection in the pFET portion of the circuit is made at the bottomlevel of the circuit, which couples the source terminal of the pFET 1614to the bottom supply (VDD) rail 1620. In the nFET portion of thecircuit, the next connection is also made at the bottom of the circuit,which couples the source terminal of the nFET 1616 to the bottom ground(GND) rail 1618.

FIG. 17 shows one example of a two-input NAND gate 1700 that comprises atop-level 1702, middle-level 1704, and bottom-level 1706. The top-level1702 comprises a top ground (GND) rail 1708, a top supply (VDD) rail1710, and an output 1712. The middle-level 1704 comprises two pFETs1714, 1716, two nFETs 1718, 1720, and two inputs 1722, 1724. The bottomlevel 1706 comprises a bottom ground (GND) rail 1726 and a bottom supply(VDD) rail 1728. Following the rules discussed above with respect toFIG. 14, the drain terminals of the pFETS 1714, 1716 are coupled to eachother and the output 1712. The first input 1722 is coupled to a firstpFET 1714 and a first nFET 1718. The second input 1724 is coupled to asecond pFET 1716 and a second nFET 1720. The drain terminal of thesecond nFET 1720 is also coupled to the output 1712. The next connectionin the pFET portion of the circuit is made at the bottom level, whichcouples the source terminals of the pFETs 1714, 1716 to the bottomsupply (VDD) rail 1728. In the nFET portion of the circuit, a connectionis made at the bottom level, which couples the nFETs 1718, 1720 to eachother. The next connection is made at the top level, which couples thesource terminal of the first nFET 1718 is coupled to the top ground(GND) rail 1708.

FIG. 18 shows one example of a two-input NOR gate 1800 that comprises atop-level 1802, middle-level 1804, and bottom-level 1806. The top-level1802 comprises a top ground (GND) rail 1808, a top supply (VDD) rail1810, and an output 1812. The middle-level 1804 comprises two pFETs1814, 1816, two nFETs 1818, 1820, and two inputs 1822, 1824. Thebottom-level 1806 comprises a bottom ground (GND) rail 1826 and a bottomsupply (VDD) rail 1828. Following the rules discussed above with respectto FIG. 14, the drain terminal of the second pFET 1816 and the drainterminals of the first and second nFETs 1818, 1820 are coupled to theoutput 1812. The first input 1822 is coupled to the first pFET 1814 andthe first nFET 1818. The second input 1824 is coupled to the second pFET1616 and the second nFET 1820. In the pFET portion of the circuit thenext connection is made at the bottom level of the circuit, whichcouples the drain terminal of the first pFET 1814 to the source terminalof the second pFET 1816. The next connection is made at the top level ofthe circuit, which couples the source terminal of the first pFET 1814 tothe top supply (VDD) rail 1810. In the nFET portion of the circuit, thenext connection is made at the bottom level of the circuit, whichcouples the source terminals of the first and second nFETs 1818, 1820 tothe bottom ground (GND) rail 1826.

FIGS. 19A and 19B show top and bottom views, respectively, of a NANDcell 1900 implementing the interconnect rules of one or moreembodiments. The cell 1900 comprises pFET regions 1902 and nFET regions1904 each comprising fins 1906; gate layers 1908, 1910; S/D contactlayers 1912; contact via 1916; and metal layers comprising a top supply(VDD) rail 1918, a bottom supply (VDD) rail 1920, a top ground (GND)rail 1922. A bottom ground (GND) rail 1924, inputs 1926, 1928, and anoutput 1930. FIG. 19A shows that the drain terminals of the pFETs arecoupled to the output 1930 at the top level. The first input 1926 iscoupled to a first pFET and a first nFET, while the second input 1928 iscoupled to a second pFET and a second nFET. FIG. 19A further shows thatdrain terminal of the second nFET is also coupled to the output 1930,and the source terminal of the first nFET is coupled to the top ground(GND) rail 1922. FIG. 19B shows that the source terminals of the pFETsare coupled to the bottom supply (VDD) rail 1920, and the two nFETs areconnected to each other.

In addition to inner-cell connections, one or more embodiments configureinter-cell (connections between multiple cells) such that inter-cellconnections do not involve any bottom level contacts. For example, allunit cells are designed and fabricated with their outputs on the toplevel. Since logic gate input is at the middle level (FET gate), bottomlevel connections are not created between different cells. However, insome embodiments, the level of outputs can be chosen such that some ofthe inter-cell connections are made at the bottom level (and below).This can save space for back-end-of-line (BEOL) processing. In thisembodiment, at least half of the inter-cell connections can be made atthe bottom level (and below). For example, FIG. 20 shows a circuit 2000comprising a plurality of different cells 2002, 2004, 2006. At least oneinter-cell connection 2008 is at the top level of the circuit, while atleast half of the remaining inter-cell connections 2010 are made at thebottom level of the circuit 2000.

FIG. 21 is an operational flow diagram illustrating one process forfabricating vertical transistors according to one embodiment of thepresent disclosure. In FIG. 21, the operational flow diagram begins atstep 2102 and flows directly to step 2104. It should be noted that eachof the steps shown in FIG. 21 have been discussed in with respect toFIGS. 1-19. A structure 100, at step 2104, is formed comprising at leasta first substrate 102, an insulator layer 104 on the substrate 102, afirst doped layer 106 on the insulator layer 104, at least one finstructure 202 in contact with the doped layer 106, a dielectric layer602 surrounding a portion of the fin structure 202, a gate layer 614 onthe dielectric layer 602, a second doped layer 802 in contact with thefin structure 202, a first contact area 902 in contact with the seconddoped layer 802, and at least a first interconnect 1004 in contact withthe first contact area 902.

The structure 100, at step 2106, is flipped. After flipping thestructure 100, the structure 100 is bonded to a second substrate 1106,at step 2108. The first substrate 102 and the insulator layer 104, atstep 2110, are removed to expose the first doped layer 106. A secondcontact area 1202, at step 2112, is formed in contact with the firstdoped layer 106. At least a second interconnect 1304, at step 2114, isformed in contact with the second contact area 1202. The control flowexits at step 2116.

FIG. 22 is an operational flow diagram illustrating one process forforming inner-cell connections according to one embodiment of thepresent disclosure. In FIG. 22, the operational flow diagram begins atstep 2202 and flows directly to step 2204. It should be noted that eachof the steps shown in FIG. 22 have been discussed above with respect toFIGS. 14-19. A sum-of-product form logic gate circuit is fabricatedcomprising parallel-connected pFET branches and series-connected nFETbranches. Each of the parallel-connected pFET branches comprisesseries-connected pFETs. Each of the series-connected nFETs comprisesparallel-connected nFETs. The process, at step 2202, begins with theparallel-connected pFET branches. A pFET branch that has not beenprocessed is identified at step 2206. The first pFET branch, at step2208, is coupled to an output at step 2208.

A determination is made, at step 2210, whether the next connection willbe to VDD. If the result of this determination is positive, the pFET iscoupled to the bottom supply rail at step 2212. Another determination isthen made, at step 2214, whether any pFET branches have not beenprocessed. If the result of this determination is positive, the controlflow returns to step 2206. If the result of this determination isnegative, the control flows to entry point A of FIG. 23. If the resultof the determination at step 2210 is negative, the next pFET is coupledto the previous pFET at the bottom level at step 2216. If the result ofthe determination at step 2210 is negative, a determination is made, atstep 2218, whether the next connection will be to VDD. If the result ofthis determination is positive, the pFET is coupled to the top supplyrail at step 2220. The control then flows to step 2214. If the result ofthis determination is negative, the next pFET is coupled to the previouspFET at the top level at step 2222. The control flow then returns tostep 2210.

The process shown in FIG. 23 starts with the series-connected nFETbranches at step 2302. The first nFET branch, at step 2304, is coupledto the output. A determination is made, at step 2306, whether the nextconnection will be to Ground. If the result of this determination ispositive, the nFET is coupled to the bottom ground rail at step 2308.The control flow then exits at step 2310. If the result of thisdetermination is negative, the next nFET, at step 2312, is coupled tothe previous nFET at the bottom level. Another determination is thenmade, at step 2314, whether the next connection will be to Ground. Ifthe result of this determination is positive, the nFET is coupled to thetop ground rail at step 2316. The control then exits at step 2318. Ifthe result of this determination is negative, the next nFET is coupledto the previous nFET at the top level at step 2320. The control flowthen returns to step 2306.

Although specific embodiments of the disclosure have been disclosed,those having ordinary skill in the art will understand that changes canbe made to the specific embodiments without departing from the spiritand scope of the disclosure. The scope of the disclosure is not to berestricted, therefore, to the specific embodiments, and it is intendedthat the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentdisclosure.

It should be noted that some features of the present disclosure may beused in one embodiment thereof without use of other features of thepresent disclosure. As such, the foregoing description should beconsidered as merely illustrative of the principles, teachings,examples, and exemplary embodiments of the present disclosure, and not alimitation thereof.

Also, these embodiments are only examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily limit any ofthe various claimed disclosures. Moreover, some statements may apply tosome inventive features but not to others.

What is claimed is:
 1. A method for forming a circuit, the methodcomprising: disposing a top supply rail and a top ground rail within atop level of a circuit; disposing a bottom supply rail and a bottomground rail within a bottom level of the circuit; disposing at least oneinput line within a middle level of the circuit; disposing an outputline within the top level of the circuit; coupling at least one p-typevertical field-effect-transistor (FET) to one of the bottom supply railand the top supply rail; and coupling at least one n-type vertical FETto one of the bottom ground rail and the top ground rail, wherein, theat least one p-type vertical FET and the at least one n-type verticalFET each comprise a bottom source/drain layer disposed below a gatestructure and a top source/drain layer disposed above the gatestructure.
 2. The method of claim 1, further comprising: coupling the atleast one p-type vertical FET to the at least one input line, the outputline, and the bottom supply rail.
 3. The method of claim 1, wherein theat least one p-type vertical FET comprises a plurality of p-typevertical FETs, and wherein the method further comprises: coupling eachp-type vertical FET of the plurality of p-type vertical FETs to the atleast one input line, the output line, and the bottom supply rail. 4.The method of claim 1, further comprising: coupling the at least onen-type vertical FET to the at least one input line and, the output line,and the bottom ground rail.
 5. The method of claim 1, wherein the atleast one n-type vertical FET comprises a plurality of n-type verticalFETs, where the plurality of n-type vertical FETs comprises an evennumber of n-type vertical FETs, and wherein the method furthercomprises: coupling each n-type vertical FET of the plurality of n-typeFETs to the at least one input line; coupling at least one n-typevertical FET to the output line; and coupling at least one n-typevertical FET of the plurality of n-type vertical FETs to the top groundrail.
 6. The method of claim 1, wherein the at least one n-type verticalFET comprises a plurality of n-type vertical FETs, where the pluralityof n-type vertical FETs comprises an odd number of plurality of n-typevertical FETs, and wherein the method further comprises: coupling eachn-type vertical FET of the plurality of n-type FETs to the at least oneinput line; coupling at least one n-type vertical FET to the outputline; and coupling at least one n-type vertical FET of the plurality ofn-type vertical FETs to the bottom ground rail.
 7. The method of claim1, wherein the at least one p-type vertical FET comprises a plurality ofp-type vertical FETs, where the plurality of p-type vertical FETscomprises an even number of plurality of p-type vertical FETs, andwherein the method further comprises: coupling each p-type vertical FETof the plurality of p-type vertical FETs to the at least one input line;coupling at least one p-type vertical FET to the output line; andcoupling at least one p-type vertical FET of the plurality of p-typevertical FETs to the top supply rail.
 8. The circuit of claim 1, whereinthe at least one p-type vertical FET comprises a plurality of p-typevertical FETs, where the plurality of p-type vertical FETs comprises anodd number of plurality of p-type vertical FETs, and wherein the methodcomprises: coupling each p-type vertical FET of the plurality of p-typevertical FETs to the at least one input line; coupling at least onep-type vertical FET to the output line; and coupling at least one p-typevertical FET in the plurality of p-type vertical FETs to the bottomsupply rail.
 9. A method for forming a circuit, the method comprising:disposing a top supply rail and a top ground rail within a top level ofa circuit; disposing a bottom supply rail and a bottom ground railwithin a bottom level of the circuit; disposing at least one input linewithin a middle level of the circuit; disposing an output line withinthe top level of the circuit; coupling at least one p-type verticalfield-effect-transistor (FET) to at least the output line and the bottomsupply rail; and coupling at least one n-type vertical FET to at leastthe output line and the bottom ground rail.
 10. The method of claim 9,wherein the method further comprises: coupling a source terminal of theat least one p-type vertical FET and a source terminal of the at leastone n-type vertical FET to the output line.
 11. The method of claim 9,wherein the method further comprises: coupling a drain terminal of theat least one p-type vertical FET to the output line.
 12. The method ofclaim 9, wherein the method further comprises: coupling a drain terminalof the at least one n-type vertical FET to the output line.
 13. Themethod of claim 9, wherein the method further comprises: coupling asource terminal of the at least one p-type vertical FET to the bottomsupply rail; and coupling a source terminal of the at least one n-typevertical FET to the bottom supply rail.
 14. A method for forming acircuit, the method comprising: disposing a bottom supply rail and abottom ground rail within a bottom level of the circuit; disposing atleast one input line within a middle level of the circuit; disposing anoutput line within a top level of the circuit; coupling a plurality ofp-type vertical field-effect-transistors (FETs) to the at least oneinput line; coupling at least one p-type vertical FET to the outputline; coupling at least one p-type vertical FET of the plurality ofp-type vertical FETs to the bottom supply rail; and coupling at leastone n-type vertical FET to one of the bottom ground rail and a topground rail.
 15. The method of claim 14, wherein the method furthercomprises: coupling the at least one n-type vertical FET to the at leastone input line and, the output line, and the bottom ground rail.
 16. Themethod of claim 14, wherein the at least one n-type vertical FETcomprises a plurality of n-type vertical FETs, where the plurality ofn-type vertical FETs comprises an even number of plurality of n-typevertical FETs, and wherein the method further comprises: coupling eachn-type vertical FET of the plurality of n-type FETs to the at least oneinput line; coupling at least one n-type vertical FET to the outputline; and coupling at least one n-type vertical FET of the plurality ofn-type vertical FETs to the top ground rail.
 17. The method of claim 14,wherein the at least one n-type vertical FET comprises a plurality ofn-type vertical FETs, where the plurality of n-type vertical FETscomprises an odd number of plurality of n-type vertical FETs, andwherein the method further comprises: coupling each n-type vertical FETof the plurality of n-type FETs to the at least one input line; couplingat least one n-type vertical FET to the output line; and coupling atleast one n-type vertical FET of the plurality of n-type vertical FETsto the bottom ground rail.
 18. The method of claim 14, wherein theplurality of p-type vertical FETs comprises an odd number of p-typevertical FETs.
 19. The method of claim 14, wherein the method furthercomprises: coupling a source terminal of the at least one p-typevertical FET to the bottom supply rail.
 20. The method of claim 14,wherein the method further comprises: coupling a source terminal of theleast one n-type vertical FET to the bottom ground rail.